Thirteen children (1.8-15.8 years of age) with brain tumors were studied with [11C]L-methionine positron emission tomography (METPET). Patients were injected intravenously with tracer before a baseline PET scan was obtained. To assess the sensitivity of [11C]L-methionine uptake to competitive inhibition, 10 patients received oral L-phenylalanine (100 mg/kg); 1 hour later, a second METPET was obtained. Subjective assessment of [11C]L-methionine uptake closely paralleled results of quantitative examination (r = 0.81). [11C]L-methionine uptake in tumor-containing brain was increased in 11 patients (mean ratio of [11C] radioactivity in tumor to normal brain: 1.5 +/- 0.57; range: 1.13-2.98). Increased tracer uptake occurred in ependymomas (3), medulloblastoma (1), and astrocytomas (5), but was less intense in low-grade tumors. L-phenylalanine reduced L-methionine uptake (25-69%) in 70% of studies. L-methionine uptake was not sensitive to competitive inhibition in brain radiation injury. Two-phase METPET is of potential value in difficult clinical situations evident in children with brain tumors, including the differential diagnosis of tumor recurrence and cerebral radiation injury.